The proposed research project will separately evaluate the effect of parathyroid hormone and vitamin D on bone. In another group of studies, the effects and interactions of the vitamin D metabolites 1,25(OH)2D3 and 24,25(OH)2D3 on bone cells, structure and dynamics shall be unravelled. Specifically the following questions will be answered: a) What is the separate and combined effect of PTH and vitamin D on bone? b) Is the effect of vitamin D and its metabolites on bone a direct one or is it mediated by changes in serum calcium and phosphate? c) What is the effect and interaction of the two vitamin D metabolites 1.25(OH)2D3 and 24,25(OH)2D3 on bone. Groups of growing inbred beagle dogs with all combinations of vitamin D and parathyroid status and calcium and phosphate supplementation as well as groups of cockerels receiving continuously administered vitamin D metabolites (1.25(OH)2D3 and 24,25(OH)2D3) will be studied. After 10 and 12 weeks bones from cockerels and dogs will be processed undecalcified and quantitative histology of bone will be done for structural, cellular and dynamic histologic parameters of bone. Serum concentrations of the various vitamin D metabolites (dogs and cockerels) and parathyroid hormone levels (dog) will be determined before, during and at the end of the study. Results of the study will provide quantitative histologic data helpful for understanding and managing various clinical problems, i.e. renal bone disease, nutritional osteomalacia, hepatic osteodystrophy, idiopathic or postoperative hyperparathyroidism and osteoporosis.